In general, living sea shell -type creatures usually have their hard parts made of calcium carbonate, or, much less commonly, calcium phosphate. Calcium carbonate occurs in two forms in these invertebrates: aragonite and calcite. Aragonite becomes less stable as temperature and pressure increase, and since those changes commonly occur during lithification of sediment, agagonite in invertebrate shells alters to a more stable form (calcite) as fossilization occurs. Thus, calcite is a common mineral in fossils, and although aragonate occurs as well, it becomes more unusual in older fossils. During fossilization further changes may occur, such that calcite and/or aragonite and/or calcium phosphate get replaced with silica, haematite, pyrite or other minerals. Sometimes, the original mineral is dissolved and not replaced, leaving a cavity. That can result in fossils existing just as molds, or as casts - where such molds become filled with other material.

Oyster shells are made of calcite, and that's probably still what your fossil Rastellum is composed of.